1. Field of the Invention
The present invention relates to a rake receiver. More particularly, the present invention relates to a rake receiver using channel estimation.
2. Description of Related Art
Wireless communication system uses radio waves to transmit messages. During the process of transmitting electric waves, the electric waves are affected by buildings or automobiles in the environment, so signals from a transmitting terminal finally reach a receiving terminal after passing through various different reflection or refraction paths, and the time for the signal reaches the receiving terminal varies depending upon a different path. Therefore, the wireless communication environment is considered as a multi-path channel. From a perspective of mathematics, it is assumed that the signal transmitted from the transmitting terminal at the time t is expressed as s(t), the received signal r(t) received by the receiving terminal at the time t may be expressed the following equation:
                              r          ⁡                      (            t            )                          =                                            ∑                              l                =                0                                            L                -                1                                      ⁢                                                  ⁢                                          h                l                            ⁢                              s                ⁡                                  (                                      t                    -                                          τ                      l                                                        )                                                              +                                    n              ⁡                              (                t                )                                      .                                              (        1        )            
In the Equation (1), L is the number of paths, τ1 is the path delay time caused by the lth path in L transmission paths, hl is the channel gain of the lth path in L transmission paths, and n(t) is the channel noise.
In the current communication system, a rake receiver is usually used in the receiving terminal, a plurality of branches (also called a plurality of fingers) therein is used to collect signals in different paths, so as to resist the signal attenuation caused by the multi-path channel. Among the current applications, the rake receiver is most commonly used in the code division multiple access (hereafter referred to as CDMA) system. FIG. 1 is a systematic block diagram of a rake receiver in a CDMA system.
Referring to FIG. 1, the rake receiver 100 includes a plurality of fingers 110_0-110_(L−1) and a combiner 140. The received signal r(t) is respectively input to delay units 120_0-120_(L−1) in the fingers 110_0-110_(L−1); and the delay units 120_0-120_(L−1) respectively delay the received signal r(t) for a time τ0-τL-1 and then output it to de-spreaders 130_0-130_(L−1). Then, the de-spreaders 130_0-130_(L−1) are used to respectively generate signal components y(τ0)-y(τL-1) for being output to the combiner 140. Upon receiving the signal components y(τ0)-y(τL-1) output by the fingers 110_0-110_(L−1), the combiner 140 makes the received signal components y(τ0)-y(τL-1) be respectively multiplied by a weight w0*-wL-1*, and then summed up, and outputs a de-spreading signal {circumflex over (x)}.
The number of the fingers 110_0-110_(L−1) is designed to be the number of the paths, the time τ0-τL-1 in the delay units 120_0-120_(L−1) are the path delay time on each path obtained after the receiving terminal has performed the channel estimation, and the weights w0*-wL-1* in the combiner 140 are the conjugate of channel gains on each path obtained after the receiving terminal has performed the channel estimation. That is to say, in the practical application, the rake receiver 100 of the receiving terminal sets each finger 110_0-110_(L−1) according to the channel estimation result, so as to collect signals of each path from the channel.
In the practical transmission, the CDMA system uses a plurality of spreading codes to transmit a plurality of user' signals at the same time, and uses the orthogonality between each spreading code to prevent the signals between the users from interfering with each other. Therefore, when the signal is interfered by the multi-path in the channel, the orthogonality between the spreading codes is also damaged, as a result, the signals between the users are mutually interfered, that is, multiple-user interference (MUI). In addition, the interference of the multi-path also causes inter-symbol interference (ISI) for the same user. However, the conventional rake receiver only considers the attenuation of the signals caused by the multi-path channel, but cannot overcome the MUI and the ISI problems.
From the perspective of mathematics, the conventional rake receiver assumes the noises n(t) in the channel as a white Gaussian noise, and only processes the signal attenuation caused by the multi-path (that is, only the path delay time τ0-τL-1 and the channel gains h0-hL-1 of the multi-path channel are considered). However, during the practical channel transmission, due to the MUI and the ISI, the noises n(t) in the channel are not the white Gaussian noise, but colored Gaussian noise. Therefore, the conventional rake receiver ignores the MUI and the ISI, such that the performance of the receiving terminal becomes relatively poor, and meanwhile the bit error rate of the signals demodulated by the receiving terminal becomes relatively large.
Recently, in order to solve the attenuation of the multi-path channel and to reduce the interference of the colored Gaussian noise of the channel, a US patent (Note [1]) and a paper (Note [2]) have proposed a generalized RAKE (hereafter referred to as G_RAKE) receiver. FIG. 2 is a systematic block diagram of a G-RAKE receiver in a CDMA system.
Referring to FIG. 2, a G-RAKE receiver 200 includes a plurality of fingers 210_0-210_(J−1) and a combiner 240. The architecture of the G_RAKE receiver 200 is similar to the rake receiver 100 in FIG. 1, except that the number of the fingers 210_0-210_(J−1) of the G-RAKE receiver 200 is J, and the value J is greater than the number of the paths L. Each time d0-dJ-1 in the delay units 210_0-210_(J−1) does not necessarily correspond to the path delay time on the transmission path, but designed by utilizing a statistical value of the maximum signal-to-noise ratio (SNR).
In addition, the weights w0*-wJ-1* in the combiner 240 are calculated by using the maximum likelihood (ML) rule. The w0*-wJ-1* can be represented by a vector w, and the value of w0*-wJ-1* is calculated according to w=Ru−1hJ, in which the superscript (−1) represents the matrix inversion operation. hJ is the channel gain acquired through channel estimation. Ru is a J×J matrix, in which the calculation process for each element value therein has been illustrated in the documents of Note [1] and the Note [2]. Ru is defined as a covariance matrix of the vector u, and the elements in the vector u are noises included in the signal output from each finger of the G-RAKE receiver.
In the document of Note [2], it is illustrated that the number J of the fingers of the G-RAKE receiver is greater than the number L of the paths of the channel, which aims at making a part of the fingers match with the paths in the channel, so as to collect the signal of each path, and also aims at making the remaining fingers (that is, the inner delay time not equal to the path delay time of the multi-path channel) whiten the colored Gaussian noises in the channel. Therefore, the G-RAKE receiver can solve the attenuation of the multi-path channel and can reduce the interference of the colored Gaussian noise of the channel.
Although the G-RAKE receiver makes the receiving terminal have relatively desirable performance, the dimension of Ru is J×J, such that a large amount of calculations is required when the inverse operation of the matrix Ru is performed. In addition, the time d0-dJ-1 designed through maximizing SNR also requires a considerably amount of calculations.
Note [1]: US Patent Publication No. US 2006/0188007 A1.
Note [2]: G. E. Bottomley, T. Ottosson, and Yi-Pin Eric Wang, “A Generalized RAKE Receiver for Interference Suppression”, IEEE J. select. Areas Commun., vol. 18, pp. 1536-1545, August 2000.